The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD) and wireline logging applications, is well known. Such techniques may be utilized, for example, to determine a subterranean formation resistivity, which, along with formation porosity measurements, may be used to indicate the presence of hydrocarbons in the formation. It is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Techniques for making microresistivity measurements of a subterranean formation are well known in the prior art for both wireline and LWD operations. Microresistivity logging tools commonly make use of one of two known measurement principles depending upon whether conductive (water based) or non-conductive (oil based) drilling fluid (mud) is being used. When conductive drilling fluid is utilized, the borehole annulus provides a good conduit for electrical current. One of the primary challenges is to focus the electrical current so that it enters the formation. The use of non-conductive drilling fluid poses different challenges. An oil based drilling fluid can severely impede the flow of electrical current through the fluid into the formation. One significant challenge is in causing the electrical current to penetrate the drilling fluid so that it enters the formation.
Those of skill in the art will understand that oil based drilling fluid is commonly utilized when drilling through water soluble formations (e.g., including salt layers). The use of oil based (non-conductive) drilling fluid is known to greatly reduce the effectiveness of microresistivity logging tools configured for use with water based (conductive) drilling fluid. Likewise, it is generally known in the art that microresistivity logging tools configured for use with oil based drilling fluid (e.g., as described in the preceding paragraph) are not well suited for making microresistivity measurements in conductive drilling fluid.
Microresistivity sensors configured for use with non-conductive drilling fluid commonly include at least four electrodes: including a pair of spaced potential electrodes deployed between current injector and return electrodes. In use, a high frequency alternating current (e.g., on the order of 1 megahertz) is passed between the injector and return electrodes. A high frequency is typically required so as to reduce the electrical impedance of the oil based drilling fluid and enable a portion of the current to penetrate the formation. The use of high frequencies is also known to cause displacement currents in the sensor and in the non-conductive drilling fluid. In the absence of these displacement currents (or when the displacement currents have been accounted for), the voltage drop between the potential electrodes tends to be approximately proportional to the resistivity of the formation.
While four-electrode configurations can be utilized in nonconductive drilling fluid, there remain significant difficulties in logging while drilling applications. For example, drilling operations make it difficult to maintain close proximity of the sensor to the borehole wall. Varying sensor standoff can significantly deteriorate the image resolution. Attempts have been made to use flexible or spring loaded pads to better control the sensor standoff, but such configurations are not generally mechanically robust in LWD operations. Therefore, there exists a need in the art for a microresistivity logging tool (and a sensor) that is suitable for use in non-conductive drilling fluid and generally insensitive to sensor standoff.